1. Field of the Invention
The present invention relates to zoom lens systems suitable for use as image-taking lenses of image pickup apparatuses such as a video camera, a monitoring camera, a digital still camera, a broadcast camera, and a silver-halide film camera.
2. Description of the Related Art
Examples of an image pickup apparatus include a video camera, a digital still camera, and a broadcast camera each including a solid-state image pickup device, and a silver-halide film camera. While the functionality of recent image pickup apparatuses is becoming higher, the sizes thereof are becoming smaller.
Image-taking lenses (image-taking optical systems) to be included in such image pickup apparatuses are desired to be compact zoom lens systems with short total lens lengths and high zoom ratios.
To satisfy such a demand, there are zoom lens systems each including four lens units: first to fourth lens units having positive, negative, positive, and positive refractive powers, respectively, in order from an object side to an image side.
Some of these zoom lens systems are of a rear-focus type in which the second lens unit is moved for zooming and the fourth lens unit is responsible for focusing and correction of image-plane variation occurring during zooming, as disclosed in, for example, US Patent Laid-Open No. 2006/0146417, U.S. Pat. No. 7,199,942, Japanese Patent Laid-Open No. 2002-182109, Japanese Patent Laid-Open No. 2002-287027, U.S. Pat. No. 6,118,593, and Japanese Patent Laid-Open No. 2000-227548.
In a zoom lens system including four lens units and employing the rear-focus method, the effective diameter of the first lens unit is small and the overall size of the lens system can be reduced easily, compared with a zoom lens system in which the first lens unit is moved for focusing.
In such a rear-focus zoom lens system, however, a large aberration variation occurs during focusing with the fourth lens unit, making it difficult to realize high optical performance over the entire object-distance range from an object at infinity to a near object.
Therefore, to realize good optical performance over the entire object-distance range while reducing the size and increasing the zoom ratio of a zoom lens system, it is important to appropriately set the zoom type, the refractive powers and configurations of the lens units, and so forth.
In the foregoing rear-focus zoom lens system including four lens units, it is particularly important to appropriately set the configuration of the fourth lens unit that moves during zooming and focusing.
If the fourth lens unit has an inappropriate configuration, aberration variation occurring during zooming and aberration variation occurring during focusing with the fourth lens unit become large, making it difficult to realize high optical performance over the entire zoom range and the entire object-distance range.